The bureau's preliminary investigation found that the waste water including cutting fluid and oil had flowed from the company's storm drain. Other environmental problems were also found including noise levels exceeding standards.

The environmental bureau has penalized the supplier, although it is unclear if the company was fined. RiTeng's parent company Casetek made its Friday statement in response to a news report that said the supplier would be forced to pay 270,000 yuan (US$43,000).

The RiTeng facilities, however, are still operating normally. The supplier is complying with the authorities' investigation, and working to resolve the problems, Casetek added.

RiTeng was previously in the news for a factory explosion that sent 61 workers to hospital in December 2011. One of the factories there is known to assemble iPads. Company workers have complained of chemical smells and unpaid overtime.

Apple, however, has been making progress in keeping its suppliers in line with environmental regulations, said Wang Jingjing, vice director for China's InstituteA of Public and Environmental Affairs (IPE). The group had previously been critical of Apple for failing to monitor its suppliers. But the company has since stepped up audits and increased pressure on suppliers to comply, she said.

"Apple has opened up and you can see they have taken a great effort to push their suppliers to take corrective action," Wang said. The group released a report last month documenting the improvements.

IPE has already informed Apple about the wastewater dumping by RiTeng, and the U.S. company is sending staff to investigate, according to Wang.